The present invention is directed to an optical duplexer for bidirectional optical information transmission and in particular to an optical duplexer having a wavelength-selective optical coupler having four ports. A first port is provided for input of a first optical wavelength and a second port is provided for input of a second optical wavelength. The coupler is fashioned such that an optical wave of the first wavelength input through the first port at a first side of the coupler is conducted partially to the second port at a second side of the coupler and partially to a third port also at this second side. The optical wave of the second wavelength input through the second port is entirely or partially conducted to a fourth port at the first side of the coupler. An optical wave of the first wavelength returning to the coupler from the second port is supplied to the coupler on a first path and an optical wave of the first wavelength returning to the coupler from the third port is supplied to the coupler on a second path. The optical waves of the first wavelength returning to the coupler on the two paths are brought to interference in the coupler and the combined optical waves of the first wavelength are entirely or partially supplied to the fourth port.
A disadvantage of prior art duplexers of this type is that discontinuities in refractive index are usually present at the second and third port, a part of the optical waves having the first wavelength supplied to these two ports from the first port via the coupler being reflected back from these discontinuities by a refractive index of the coupler. In the coupler, the two waves reflected back are brought into interference and are supplied in this form as undesired light to the fourth port, producing undesired near-end crosstalk. The optical waves of the first wavelength returning from the second and third ports usually travel back to the coupler over two different optical paths that can have the same or different optical path lengths.
The near-end crosstalk produced by the afore-mentioned undesired light is dependent on a phase difference between the optical waves of the first wavelength returned over the two paths. It is known in the prior art to minimize this crosstalk by setting this phase difference to a predetermined value, whereby it was also known to generate this predetermined value on the basis of a correspondingly selected difference in path lengths for the two optical paths. This passive phase matching requires technologies that must be governed with extreme precision for producing the duplexer that is preferably manufactured in integrated-optical form. In such an integrated-optical duplexer, the value of the difference in path length requires that the optical path length be governed to fractions of the wavelength, resulting in geometrical tolerances of approximately 20 nm.